1. Field of the Invention
This invention relates to lift arm structures for hydraulic load lifting vehicles and particularly to a lock down apparatus and method for securing one or more lift arms of a front end loader or boom lift in a lowered position.
2. Brief Description of the Prior Art
Front end loaders and boom lifts are generally operated through control levers, which actuate hydraulic control valves governing the supply of pressurized hydraulic fluid to various hydraulic cylinders and, in the case of hydraulically driven loaders and lifts, hydrostatic drives. These control levers are generally all placed within the operator's compartment and close to the operator's seat for easy access by the operator. As a result, it frequently occurs that an operator of such a hydraulic front end loader or boom lift, when getting into or leaving the operator's compartment, accidentally moves one or more of the control levers. Furthermore, as long as the motor of a hydraulic front end loader is running, the hydraulic liquid is normally maintained under pressure by a hydraulic pump driven by the motor. This means that, if the operator accidentally operates a control lever of the vehicle when exiting or entering the operator's compartment while the motor is running, the vehicle or parts thereof may be set into motion, which may lead to serious injury to the operator. This is especially the case in front end loaders of very compact construction such as, for example, skid steer loaders. In these loaders, the operator must climb over the lift arms of the loader boom or an implement affixed thereto in order to enter or exit the operator's compartment. Thus, an apparatus is required for front end loaders and boom lifts which secures at least one lift arm or boom in a lowered, preferably a ground engaging position, thereby preferably also preventing the loader vehicle from running away, especially since the operator may be required to repeatedly exit and re-enter the operator's compartment for maintenance and servicing of the machinery. The operator may also want to remotely operate the machinery or to operate attached equipment such as a backhoe from a separate operating station. If a loader type vehicle is equipped with a fork mounted on the lift arm or boom thereof for use of the vehicle as a fork lift, it is desirable, as a safety precaution, to lock the fork in a lowered position during manual loading of the fork. It may also be desired to lock the lift arm or an implement mounted thereon in ground engaging condition as a theft deterrent.
A bulldozer and backhoe boom lock down device is known from U.S. Pat. No. 4,373,851 by Confoey, which teaches a lock device including a split cylindrical sleeve, for installation around an extended cylinder rod of a hydraulic backhoe boom or bulldozer blade lifting cylinder. The sleeve may be locked in the installed condition. The sleeve is installed after the backhoe boom or bulldozer blade is forced against the ground by the lifting cylinder and prevents retraction of the cylinder rod into the cylinder, thus preventing the theft of the backhoe or bulldozer. It is a disadvantage that such a lock device cannot be employed for locking down a lift arm of a loader type vehicle, since the device may only prevent retraction of a hydraulic cylinder rod. Lift arm control cylinder rods of loader vehicles must generally be prevented from extending rather than retracting, if the lift arm is to be secured in a lowered position. It is a further disadvantage of such a locking device that the operator cannot install the locking device from within the operator's compartment, so that the operator is not protected when exiting the operator's compartment.
These disadvantages are overcome with a lift arm lock down apparatus and method in accordance with this invention, which provides for the locking of the lift arm of a front end loader or boom lift to the vehicle body in a lowered position of the lift arm. Thus, raising of the lift arm is substantially prevented. Furthermore, forward or backward movement of the vehicle is substantially prevented, if the lift arm or an implement attached thereto has been brought into engagement with the ground before being locked to the vehicle body. It is an advantage of a lift arm lock down apparatus in accordance with this invention, that it may be employed independently of whether the lifting cylinder rod of a lift arm to be locked down must be prevented from retracting or extending, since the lock down apparatus is independent of such a lifting cylinder rod and connects the lift arm itself with the vehicle frame or body.